The present invention is related to digital communication systems and more particularly to systems and methods for transmitting system configuration information between nodes of a communication system.
Orthogonal Frequency Division Multiplexing (OFDM) systems offer significant advantages in many real world communication systems, particularly in environments where multipath effects impair performance. OFDM divides the available spectrum within a channel into narrow subchannels. In a given so-called xe2x80x9cburst,xe2x80x9d each subchannel transmits one data symbol. Each subchannel therefore operates at a very low data rate compared to the channel as a whole. To achieve transmission in orthogonal subchannels, a burst of frequency domain symbols are converted to the time domain by an IFFT procedure. To assure that orthogonality is maintained in dispersive channels, a cyclic prefix is added to the resulting time domain sequence. The cyclic prefix is a duplicate of the last portion of the time domain sequence and is appended to the beginning. To assure orthogonality, the cyclic prefix should be at least as long as the duration of the impulse response of the channel.
To maximize the performance of an OFDM system, it is desirable that the response of the channel be known at the receiver end of the link. To provide the receiver with knowledge of the channel response, the transmitter typically includes training symbols as part of the frequency domain burst. The training symbols have known values when transmitted and their values as received may be used in determining the channel response. Most of the remaining symbols are used to transmit data.
Successful communication requires a shared understanding of certain system parameters between the transmitter and the receiver. The transmitter and receiver must agree on the symbol constellation used to communicate. If channel coding is used to increase reliability, the transmitter and receiver need to agree to certain parameters. For example, if a Reed-Solomon code is to be used, both transmitter and receiver should agree as to the number of parity bytes to be included in each Reed-Solomon codeword. For convolutional or trellis encoding schemes, the transmitter and receiver must agree to the particular code being used including the so-called encoder rate.
Such parameters may be predetermined as part of the system design. It is, however, useful to vary some configuration parameters to adapt to varying data communication requirements and/or channel conditions. One could have the transmitter determine the parameters to use and then have the receiver attempt to discover the parameters being used by the transmitter by analysis of the received signal. This however requires a relatively large amount of computation time at the receiver and a substantial increase in receiver complexity and loss of robustness.
In the OFDM context, it is known to explicitly send communication configuration parameters from a transmitter to a receiver by reserving extra symbols besides the training and data symbols in each frequency domain burst for that purpose. This is the technique employed in the Digital Video Broadcasting (DVB) adopted in Europe for terrestrial video broadcasting. Each OFDM frequency domain burst then includes both periodically spaced training symbols and randomly spaced system configuration symbols.
The overhead required to transmit system configuration information reduces the available capacity for transmitting data. What is needed are systems and methods for transmitting system configuration information without affecting the amount of data that may be transmitted.
Systems and methods for transmitting configuration information in an OFDM communication system are provided by virtue of the present invention. In one embodiment, within a frequency domain burst certain symbols allocated for use in channel estimation also carry system configuration information. The system configuration information may be encoded as phase differences between corresponding training symbols of successive bursts.
According to a first aspect of the present invention, transmitter apparatus is provided for use in a digital communication system employing orthogonal frequency division multiplexing. The transmitter apparatus includes: a training information formation block that develops a first set of training symbols and a second set of training symbols. Communication configuration information is encoded as phase differences between training symbols of the first set and corresponding symbols of the second set. The apparatus further includes a training symbol/data symbol selector that forms a series of OFDM frequency domain bursts including the first set of training symbols and OFDM frequency domain bursts including the second set of training symbols. The series of OFDM frequency domain bursts is transmitted from a first node to a second node.
A second aspect of the present invention provides receiver apparatus for use in a digital communication system employing orthogonal frequency division multiplexing. The receiver apparatus includes: a transform block that provides first and second OFDM frequency domain bursts based on received signals, a data/training separation block that extracts a first set of training symbols from the first OFDM frequency domain burst and a second set of training symbols from the second OFDM frequency domain burst, a differential decoder that obtains phase differences between symbols of the second set of training symbols and the first set of training symbols, and a control processor that determines communication configuration information based on the phase differences.
A third aspect of the present invention provides transmitting apparatus for use in a digital communication system employing orthogonal frequency division multiplexing. The apparatus transmits configuration information from a first node to a second node. The apparatus includes a burst formation system that forms a frequency domain burst. The burst includes data symbols and training symbols. The training symbols also encode configuration information. The burst also includes a transform block that converts the frequency domain burst into a time domain burst for transmission from the first node to the second node.
A fourth aspect of the present invention provides receiving apparatus for use in a digital communication system employing orthogonal frequency division multiplexing. The apparatus receives configuration information. The apparatus includes: a converter that converts a received time domain signal into a series of OFDM frequency domain bursts, a selector that extracts training symbols from the series of OFDM frequency domain bursts, a channel estimation block that estimates channel response based on the training symbols, and a control processor that determines system configuration based on the training symbols.
A further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.